Weaken GRANDPA proof validation

[vmarkushin]

verify_with_voter_set function is performing additional validation that go beyond the standard implementation (section 2.3), possibly leading to proofs not being accepted while valid.

Instead, we need to accept all finality justifications where is_valid is true. The equivocations or other misbehaviors detected in the justifications must be handled too but not should not reject the proof.

[kirillt]

Related discussion for reference: https://substrate.stackexchange.com/questions/8701/grandpa-finality-proofs-validation-and-equivocations